Method of electroslag casting of ingots

ABSTRACT

A method for the electroslag casing of ingots which comprises the batchwise feeding of liquid metal into a mould. The weight of each batch amounts to from 5 to 25 percent by weight of the ingot. Each subsequent batch is fed onto the liquid surface of the preceding batch after more than half of the preceding batch has been crystallized.

United States Patent [191 Paton et al.

[45] Apr. 30, 19

[ METHOD OF ELECTROSLAG CASTING OF lNGOTS [76] Inventors: Boris Evgennievich Paton, ulitsa Kotsjubskogo, 9, kv. 21; Daniil Andreevich Dudko, pereulok Mechnikova, 3, kv. 7; Kim Kondratievich Prokhorenko, ulitsa Kurskaya, 8a, kv. 26; Jury Vadimovich Latash, Vozdukhoflotsky prospekt, 87, kv. 14; Leonid Gustavovich Puzrin, ulitsa Solomenskaya, 16, kv. 58;

I Alexei Efimovich'Voronin, ulitsa Topoleva, 3, kv. 40, all of Kiev; Evgeny Stepanovich Bondarenko, ulitsa Pervomaiskaya, 24, kv. 6, Elektrostal; Alexandr Borisovich 1 Vernik, prospekt Lenina, 30/13, kv. 54, Elektrostal; Leonid Alexeevich Kamensky, ulitsa Sovetskaya, 1/ 104, kv. 61, Elektrostal; Alexandr Borisovich Mostovoi, prospekt Lenina, 29, kv. 49, Elektrostal; Vladimir Vlasovich Golovchenko, ulitsa Pushkina, 29, kv. 49, Elektrostal; Anatoly Vasilievich Demidov, prospekt Lenina, 28, kv.

41, Elektrostal; Evgeny Tikhonovich 1 Dolbenko, ulitsa Garibaldi, 10, kv. 93, Moscow; Sergei Mikhailovich Filippov, ulitsa Dybenko, l0, korpus 5, kv. 406, Moscow; Vladimir Semenovich Dub, ulitsa v Festivalnaya, l4, korpus 3, kv. 27, Moscow, all of USSR.

22 Filed: Sept. 27, 1972- 21 Appl. No.: 292,831

FOREIGN PATENTS OR APPLlCATIONS 1,280,506 10/1968 Germany Primary ExaminerJ. Spencer Overholser Assistant Examiner-John E. Roethel Attorney, Agent, or Firm-Holman &-Stern [57] ABSTRACT A method for the electroslag casing of ingots which comprises the batchwise feeding of liquid metal into a mould. The weight of each batch amounts to from 5 to 25 percent by weight of the ingot. Each subsequent batch is fed onto the liquid surface of the preceding batch after more than half of the preceding batch has been crystallized.

5 Claims, No Drawings METHOD OF ELECTROSLAG CASTING OF INGOTS from oxidation, the mould is preliminarily filled with slag or a slag mixture, whereafter the melt is introduced into the mould, the metal ingot. being formed beneath a slag blanket.

It has previously been proposed to feed the liquid metal into the mould in a'batchwise manner, the mould being filled beforehand with a slag mixture, with each batch of the metal subsequent to the first one being fed after the horizontal surface of the preceding batch has been covered with a skin of hard metal whose temperature is higher than the melting point of the slag mixture but lower than the crystallization temperature of the metal being poured on the mixture.

In the batchwise feeding of the molten metal into the mould by using the above technique, each subsequent batch of the melt is introduced after the lateral faces of the ingot have been covered with a skinof hard metal sufficiently strong to withstand the stresses which might arise in withdrawing the ingot from the mould. However, with that method different batches of the metal being poured, though having one and the same chemical composition, are not securely welded together, the foregoing defect being expecially manifestat mould walls. In addition, the above method does not ensure the formation of a dense structure in the central portion of a heavy ingot. t

A method for the electroslag casting of metal ingots has been proposed, which comprises establishing a slag bath in a mould (either by pouring liquid or by'melting hard slag) by using nonconsumable (graphite) electrodes.

Subsequently the requisite temperature of the slag bath produced thereby is maintained by electrical cur.- rent supplied to the electrodes. In this case the molten metal is fed into the mould in a continuous flow. As the level of the metal and slag goes up, the electrodes heating an ingot head are raised (see Authors Certificate of the USSR No. 168743).

' With the above method the slag not only protects the surface of the meltbut also provides for its refining due to a reduction in its sulphur and oxide contents and non-metallic inclusions with the ensuing enhancement of the quality of the ingot surface.

However, it is impossible to feed the liquid metal continually into the mould with a very low speed commensurable with its crystallization rates ensured in remelting consumable electrodes- The molten metal is therefore introduced into the mould at the rates previously employed'in pouring steel. 7

With the prior art methods for the electroslag casting of ingots, the crystallization of the major fraction of the metal poured into the -.mould initiates and proceeds when the mould is filled up with the metal. Therefore Hence, the methods thus far proposed for the pro duction of metal ingots by the batchwise feeding of liquid metal into amould, containing a slag mixture, on a skin of the crystallized preceding batch, and electroslag casting or pouring by establishing a slag bath with the aid of electrodes and pouring the metal through that bath, do not make it possible to obtain high quality ingots, especially heavy ones which are needed for rotors of high power turbinegenerators in a single unit.

The previous-methods of casting heavy ingots do not provide for high physical and chemical homogeneity of the ingots as a whole nor do they provide for crystal growth directed from the bottom upward.

The basic object of the invention is to enhance the quality of metal ingots, preferably heavy ones for subsequent forging, by providing the conditions for the growth of crystals directed from'the bottom upward along with high physical and chemical homogeneity of the ingot bothover its height and across its crosssection.

Still another object of the invention is to provide an ingot having a high quality surface formed by pouring liquid metal through slag kept overheated by electrical current supplied to the electrodes.

Yetanotherobject of the invention is to provide metal refining during the casting operation due to a reduction in sulphur content and in non-metallic inclusions under the effect of slag.

The above objects are achieved by developing a method for the electroslag casting of ingots by the batchwise feeding of liquid metal into a mould wherein a slagbath is preliminarily heated by electrical current Such volume of the metal batches being poured along with their heating on the side of the metal feed ensure the crystallization of the liquid metal directed from the bottom upward. When thenew batches of the molten metal are poured, they are mixed with the metal not yet crystallized and are enriched in liquation admixtures providing greateruniformity of the chemical composition of successive batches. Preservation of the metal the incidence of the defects related to liquation and bath at the surface of the ingot being formed induces good feeding of the central sections of the ingot, all this being conducive to the building-up of the ingot featuring a more uniform composition.

The poured batch of the metal is preferably heated at the mould walls through the slag by means of the electrodes being located at the circumference of the mould.

The heating of the circumferential portion of the metal bath will preclude the appearance at the mould walls of both a slag skin and a metalshell impairing the formation of the ingot surface; After the last batch of the metal has been poured into the mould, the electrodes may be moved toward the central portion of the horizontal surface of the ingot metal bath.

The above heating will contribute to a lesser depth of the metal bath in the axial part of the ingot, also ensuring pronounced vertical crystallization of the ingot.

It is preferable to employ three pairs of electrodes for heating the poured metal batch through the slag, feeding the electrodes from a three-phase A.C. current source. In this manner, the electrodes may be better distributed around the ingot circumference and provide better heat transfer to the metal at the mould walls through the slag thereby promoting better formation of the ingot.

It is also possible to reversibly revolve the mould together with the ingot being cast therein about its longitudinal axis through an angle ranging within 30-l20. Such revolving of the mould will ensure a more uniform distribution of the heat along the periphery of the ingot, contributing thereby to better formation of the ingot surface.

The batches of the molten metal preferably constitute separate heats obtained in at least a single furnace, the weight of each heat being less than that of the ingot.

If the batches constitute the fractions of a heat produced in one and the same furnace, the temperature of the metal remaining in the furnace as the metal is being tapped from it shall be kept at the requisite level and its chemical composition shall be possibly adjusted to compensate for the potential burning-out of alloying elements.

If the batches constitute the entire heats, in order to secure a uniform chemical composition of the ingot being cast, provision shall be made to provide a minimum difference in the chemical composition of the heats manufactured separately in a single or in several furnaces by standardizing both the charge and the technology of melting.

The second method of operation is more preferable. It enables the production of the ingots of any predetermined weight by using a furnace (or furnaces) of relatively small capacity.

The nature of the present invention will be more clear from the following description of an exemplary embodiment conforming to the invention.

In manufacturing heavy ingots to be subjected to subsequent forging, for example, 190 t in weight, a watercooled mould about 2,500 mm in diameter and 5,000 mm high is mounted on a water-cooled base plate, on which is preliminarily placed a dummy bar in the form of a steel washer, its chemical composition being similar to that of the ingot being cast. Introduced into the mould until their lower ends come in contact with the dummy bar are, for instance, six graphite electrodes. Each electrode is 200-400 mm in diameter. The three pairs of electrodes are fed from a three-phase commercial frequency current source.

The electrodes are uniformly distributed over the circumference of the mould cross-section, separated from its walls at a distance of about 100-300 mm. The mould is filled with either a slag having high refining capacity or a mixture of charge constituents of such a slag. Applied across each pair of the electrodes is a voltage of about 50-90 V fed from a single three-phase or three single-phase transformers, and the slag is melted off. This results in a layer of a liquid slag or a slag bath being formed in the mould and an electroslag process of heating the bath is initiated. Molten slag starting is also possible by pouring into the mould the liquid slag melted especially for that purpose. Next the mould is filled with the first batch of steel, t in weight, melted beforehand in a steel-melting electric-arc furnace. The

layer of the metal formed in the mould is about 250 mm thick (0.1 ingot diameter).

Each batch of the molten metal subsequent to the first one is fed to the liquid surface of the preceding batch after more than half of the preceding batch has been already crystallized. The batches amount to from 5 to 25 percent by weight of the ingot. They constitute separate heats produced in at least a single furnace, the weight of each heat being less than that of the ingot.

As the mould is filled up with steel, the electrodes are shifted upwards, the current value in each electrode being maintained within from 10,000 to 20,000 A.

Within 4-5 hours (the time interval required for the next heat to be finished in an electric-arc furnace) the second batch of the metal, about 10 t in weight, is poured into the mould through the slag onto the liquid surface of the preceding batch, more than half of which has been crystallized. The batchwise pouring operation is repeated 19-20 timesto produce an ingot of the specified (190 t) weight.

In holding the metal batches in the mould between the two subsequent pouring operations, the requisite electrical regime is employed to keep the metal at the mould walls in a liquid state. The latter is achieved by mounting the electrodes near the mould walls, increasing their number and by turning the mould about its longitudinal axis through an angle ranging within from 30 to After-the last batch of the melt has been poured, the input power fed to the slag bath is progressively decreased, and the electrodes are traversed to the mould axis to ensure the crystallization of the metal bath without producing a shrinkage hole.

The heretofore described method of the electroslag casting of ingots is particularly useful for the production of high quality heavy ingots designed for subsequent forging and ranging in weight from over 40 t and up to 200-300 t, which are required for the manufacture of rotors of high power turbines in a single unit.

High quality of heavy ingots produced by the above technique is attained not only by electroslag treatment of steel in a mould and its directed crystallization, but by the fact that they can be produced from electric-arc steel, i.e. from a steel more clean (in terms of its sulphur and oxygen contents and non-metallic inclusions) than an open-hearth steel which has previously been employed for casting heavy ingots to be subjected to subsequent forging. Prior to being poured into the mould the metal may undergo processing by synthetic slags and by blasting with inert gases.

In another embodiment of the above method an electrode holder might be revolved about the longitudinal axis of the mould which remains stationary.

What is claimed is:

l. A method for the electroslag casting of ingots comprising the steps of: establishing a slag bath in a mould; heating said bath by electrical current fed by electrodes; feeding liquid metal into said mould in a plurality of batches, the first batch being fed on said slag bath in said mould and each subsequent batch being fed on the surface of the liquid metal of the preceding batch after more than half of the preceding batch has been crystallized, each of the batches ranging within from 5 to 25 percent by weight of the ingot and constituting separate heats obtained in at least a single furnace.

2. The method of claim 1 in which said liquid metal is heated at the walls of said mould through said slag by of electrodes which are fed from a three-phase A.C.

current source.

5. The method of claim 1 in which said mould with the ingot being cast therein is reversibly revolved about its longitudinal axis through an angle ranging from 30 to 

1. A method for the electroslag casting of ingots comprising the steps of: establishing a slag bath in a mould; heating said bath by electrical current fed by electrodes; feeding liquid metal into said mould in a plurality of batches, the first batch being fed on said slag bath in said mould and each subsequent batch being fed on the surface of the liquid metal of the preceding batch after more than half of the preceding batch has been crystallized, each of the batches ranging within from 5 to 25 percent by weight of the ingot and constituting separate heats obtained in at least a single furnace.
 2. The method of claim 1 in which said liquid metal is heated at the walls of said mould through said slag by said electrodes set up over the circumference of the mould.
 3. The method of claim 2 in which upon pouring the last batch of the metal into said mould said electrodes are moved toward the central portion of the horizontal surface of the ingot metal bath.
 4. The method of claim 2 in which the heating of the liquid metal through the slag is effected by three pairs of electrodes which are fed from a three-phase A.C. current source.
 5. The method of claim 1 in which said mould with the ingot being cast therein is reversibly revolved about its longitudinal axis through an angle ranging from 30 to 120*. 